. The objective is to develop a novel optical system for in vitro assay of microbial metabolism. It uniquely presents opportunities to assay for the presence or absence of viable organisms, monitor microbial activity, and conduct tests with microbes for toxic or stimulatory conditions. The method measures oxygen consumption in samples. It is practical for conducting rapid, economical analyses of multiple small samples. The approach innovatively uses oxygen sensitive indicators in assay samples. Spectral properties of the indicator change in an oxygen- concentration dependent manner. The oxygen content of samples is determined by interrogation of indicators in vessels using an external optical reader. The aims of Phase II are to increase the sensitivity of the assay system, improve the sensing design, and adapt the technology to the assay of small samples. Investigations will be conducted to establsih its value in bioassays and testing sterility of samples in less time than conventional techniques. The system will be versatile and useful for numerous applications ranging from sterility tests, toxicity testing, and microbial research, to therapeutic drug development. It has widespread potential for use in assays that involve microbes, or testing for microbes in diagnostic, research, environmental and many commercial applications.